Many trucks, trailers and buses which are equipped with air brakes are also equipped with at least a pair of emergency brake actuators which normally hold these respective brakes in their open position by air pressure, such actuators being provided with springs which set their brakes should the air pressure drop below a critical pressure either by a break or leak in the air line or due to malfunction of the compressor. Usually each actuator is provided with a bolt or the like for releasing its emergency brake. This requires one to crawl under the vehicle and engage the bolt with a wrench; first, however, the vehicle must be blocked to prevent movement when the emergency brakes are released.
The brake failure, which has caused the emergency brakes to set, usually occurs on a highway or freeway where the presence of the stalled vehicle usually constitutes a hazard and delay in removing the vehicle increases the hazard. Usually, in order to release the brakes, it is necessary to wait for the arrival of a tow truck operator, thus greatly prolonging the period of time the disabled vehicle remains as a hazard.